1. Field of the Invention
The present invention relates to an image processing method for binary digitizing multi-level data, and an image processing apparatus utilizing said image processing method.
2. Related Background Art
For binary digitization of multi-level image data, there is already known a dither method, in which an m.times.n dither matrix (m, n being natural numbers) is prepared, and the multi-value data are compared with corresponding threshold values in the matrix to form a binarized m.times.n matrix, thereby reproducing a halftone image in a pseudo manner. However such dither method can only reproduce m.times.n+1 density levels, and is also limited in resolving power. On the other hand, the error diffusion method, proposed in 1975 in an article "An adaptive Algorithm for Spatial Grayscale" in SID DIGEST, is superior in the resolving power and the tonal rendition, and is attracting attention in recent years.
In said error diffusion method, the binary digitizing is conducted with a fixed threshold value, and the difference between a corrected density, obtained by adding the error diffused from the past pixels to the density of an object pixel and the binarized density (0 or 255 in case of 8 bits) is diffused as a new error to forward pixels.
However the binary digitizing with said error diffusion method results, in the presence of a low density area in the initial part of digitizing, in the formation of a white area without any dot in such low density area.
A similar phenomenon is encountered in the low density area in the vicinity of edges of an image.
Also in a uniform low density area, the dots are not distributed uniformly, and the reproduced image gives an unpleasant granular feeling.
Furthermore, in a uniform density area, there are generated textures in the image, and sometimes pseudo contours.
In order to prevent such drawbacks in the error diffusion method, certain proposals have been made in U.S. patent application Ser. No. 270,809 filed Nov. 14, 1988, U.S. patent application Ser. No. 319,057 filed Mar. 6, 1989 and U.S. patent application Ser. No. 396,339 filed Aug. 21, 1989.
These inventions obtain an image of high quality, but require a large amount of two-dimensional calculations in the correction of error between the input image data and the output image data, whereby the hardware structure becomes very complicated.
Also there have also been proposed binary digitizing methods capable of providing an image equivalent to that obtainable in the error diffusion method with a reduced amount of processing, in U.S. patent application Ser. No. 476,766 filed Feb. 18, 1990, Ser. No. 514,616 filed Apr. 26, 1990 and Ser. No. 515,222 filed Apr. 27, 1990. These inventions are based on determining a binarizing threshold value by applying predetermined weighting to plural binarized data and by determining the average density around the object pixel, effecting the binary digitization utilizing said threshold value, and diffusing the error generated in said digitization into pixel data not yet binarized. These methods, relying on binary digitization based on the average density, can reduce the number of pixels into which the error is diffused in comparison with the error diffusion method, thereby significantly reducing the amount of processing.
However, the image processed by these methods is associated, as in the case of said error diffusion method, with drawbacks such as formation of white dots, granular noises or pseudo contours.